For example, a conventional calculation for inhibitor diffusion in post exposure bake resist is disclosed in J. Crank. "The Mathematics of Diffusion", Clarendon Press, pp. 11-13 (1975).
When inhibitor diffusion in baking is isotropic and the diffusion coefficient is not dependent on concentration and diffusion time, the inhibitor concentration distribution m after time t is given by: EQU m={m.sub.0 /(4.pi.Dt).sup.3/2 }exp{-(x.sup.2 +y.sup.2 +z.sup.2)/4Dt}(1)
where D is the diffusion coefficient and m.sub.0 is an initial inhibitor concentration.
When the initial inhibitor concentration distribution m.sub.c (x,y,z) before baking is given by expression (1), the concentration distribution (x,y,z) after the inhibitor diffusion due to baking for time t is given by: EQU m(x,y,z)=m.sub.0 (x',y',z')g(x-x',y-y',z-z')dx'dy'dz' (2)
where g is gaussian distribution, which is given by: EQU g(x,y,z)=[1/{2.pi.(2.pi.).sup.1/2 .sigma..sup.3 }]exp{-(x.sup.2 +y.sup.2 +z.sup.2)/2.sigma..sup.2 } (3)
where .sigma. is diffusion length and is given by .sigma.=(2Dt).sup.1/2.
The integration of expression (2) is calculated in the range of three times (3.sigma.) the diffusion length.
However, in the conventional post exposure bake simulation method, there is a problem that, when the diffusion length is elongated, the calculation time becomes long as the number of integration points is increased.